1. Technical Field
The present application relates to a micro-pilot injection ignition type gas engine and an air fuel ratio control method thereof. The gas engine of the mentioned type inhales pre-mixed air fuel mixture into each cylinder, namely, into a main chamber, and initiates combustion of the mixture inside the main chamber with at least one pilot flame that jets out from a pre-chamber through at least one nozzle hole made in a pre-chamber housing, as an ignition-source for the mixture inside the main chamber; whereby the pilot flame jet is induced by means of combustion of an amount of micro-pilot fuel-oil that is injected through a pilot fuel injector as well as by means of combustion of a part of the air fuel mixture inhaled inside the pre-chamber from the main chamber through at least one nozzle hole. In the gas engines of the mentioned type, this application relates to an air fuel ratio control device that performs an open-close control as to a gas valve for each cylinder.
2. Description of the Related Art
In conventional micro-pilot injection ignition type gas engines, pre-mixed air fuel-gas mixture of lean gas concentration is supplied into each main combustion chamber; the mixture is ignited by an ignition device such as a micro-pilot ignition mechanism with a fuel-oil injector; as a result, at least one ignition flame jet for igniting the air fuel mixture in the main chamber is induced. Thus, the starting mechanism of the micro-pilot injection ignition type gas engines is complicated. Therefore, in starting the gas engine of the type, smooth combustion transition from a starting stage to a load operation stage is not so easy to be secured in comparison with other gas engines of different types. Thus far, much effort has been made to improve the starting performance as to the micro-pilot injection ignition type gas engine.
FIG. 5 shows an example of a flow-chart diagram as to a starting method of a gas engine with a micro-pilot ignition system, according to a conventional technology.
As a first step (Step 1) in FIG. 5, a start order is transmitted to the engine; then, the engine is started with a ½ skip-firing intermittent operation (Step 2). As shown in FIG. 3-1, during the ½ skip-firing intermittent operation, each cylinder alternates a cycle with firing and a cycle without firing; that is, each cylinder repeats a cycle with combustion and a cycle without combustion alternately; in addition, Numerals 0 and 1 in the table of FIG. 3-1 correspond to the cycle without firing and the cycle with firing, respectively.
In the case of a ⅕ skip-firing intermittent operation, each cylinder repeats a cluster of every five consecutive cycles with a non-firing cycle and four consecutive firing cycles as shown in the table of FIG. 3-2; thereby, Numerals 0 and 1 also correspond to the cycle without firing and the cycle with firing, respectively.
Besides the ½ skip-fring intermittent operation or ⅕ skip-firing intermittent operation, there can be another type of intermittent operation mode, such as a n/m skip-firing intermittent operation during which n non-firing cycles (skipped cycles) exist within m consecutive cycles. Various kinds of intermittent operations are collectively cited with a term “skip mode” or “skip-firing mode” in this specification. Specifically, the terms ½ skip-firing intermittent operation and ⅕ skip-firing intermittent operation in the above description are generalized, for example, with terms “a first skip mode” and “a second skip mode” respectively. Further, when n is equal to 1, then a 1/m skip-firing intermittent operation or a 1/m skip-firing mode can be defined; whereby, a non-firing cycle is placed between m−1 consecutive firing cycles and m−1 consecutive firing cycles; thereby, a ratio of valve-open-frequency to valve-closed-frequency increases, as the number m increases. Hereafter, this ratio is called a valve opening-ratio. In addition, m consecutive engine cycles are often called a cluster of cycles in this specification.
Secondly, the speed of the engine is increased (Step 3) to an 80% speed (80% N), that is, a prescribed idling speed of a rated speed N, while the ½ skip-firing intermittent operation is performed; at this stage, a timer is activated so that the engine continues the 80% N idling for three minutes (Step 4 and Step 5); then, the engine speed is increased to a 90% speed (90% N) of the speed N (Step 6); further, the ½ skip-firing intermittent operation is shifted to the ⅕ skip-firing intermittent operation (Step 7), namely, fuel supply frequency is increased as a state of one firing-skip cycle during two consecutive cycles is shifted to a state of one firing-skip during five consecutive cycles; then (Step 8), the engine speed is increased to 100% of the rated speed N (100% N).
Thirdly, when 10% load of the rated engine-load is detected (Step 9) during the speed of 100% N, the engine is put under normal speed operation without a skip-firing mode.
A patent reference JP1997-14057 discloses a fuel supply device for starting a gas engine, the engine comprising:
a first fuel passage toward each main combustion chamber (each cylinder);
a second fuel passage directly toward a pre-chamber, the second passage being provided with a slow-open valve that begins to open the moment the engine is started, while the opening of the valve is increased in proportion to elapsed time;
whereby an air fuel ratio inside the pre-chamber is held in a flammable range, for a predetermined time span so that the engine starting performance is improved.
However, the mentioned conventional technologies involve the following subject to be solved.
Specifically, in the four-stroke cycle gas engines, fuel-gas and air are premixed and supplied to each main combustion chamber; further, in micro-pilot injection ignition type gas engines, at least one flame jet produced by the mentioned micro-pilot ignition system ignites the premixed air fuel mixture in the main chamber, and a lean-burn is performed; therefore, in the conventional gas engines of the type, engine speed fluctuations with unstable behavior are prone to occur, while the engine is in its starting process.
A prerequisite to secure an ignition performance and a starting performance as to the four-stroke cycle gas engines is to bring the air fuel ratio of the air fuel mixture in the main chamber close to a stoichiometric mixture ratio as to the fuel-gas being used; however, when the engine is started and the speed is low, the fuel-gas supply rate is low, while the air supply rate is large because of relatively ample piston pumping work; thus, the air fuel ratio during starting the engine is too lean and is in fact considerably below the stoichiometric ratio.
To increase such a lean air fuel ratio, in conventional spark ignition gas engines with a sparkplug in each cylinder, a throttle (an air fuel ratio controller) is usually provided in the air passage so as to lessen air supply; however, it is difficult to introduce such a manner into the micro-pilot injection ignition type gas engines, because such a throttle (an obstacle) in the air passage of engines of the type is not conducive to producing relatively higher output and enhanced performance in normal operation.
Thus, a conventional gas engine of a micro-pilot injection ignition type controls air fuel ratios in starting by means of enhancing fuel-gas flow rates, i.e. fuel-gas supply pressure pulsation through, while the ½ skip-firing intermittent operation and the ⅕ skip-firing intermittent operation are incorporated according to FIG. 5.
However, in the method itself according to FIG. 5, a series of the starting steps are routinely executed without feedback of engine operation conditions; namely, after a start order is transmitted to the engine, the engine is started with the ½ skip-firing intermittent operation; the speed of the engine is increased to 80% of the rated speed N (80% N); at this stage, a timer is activated so that the engine continues at 80% N for three minutes; then, the engine speed is raised to 90% of the speed N (90% N); at which speed the ½ skip-firing intermittent operation is changed to the ⅕ skip-firing intermittent operation; then, the engine speed is raised up to 100% of the rated speed N (100% N).
Thus, until the engine speed is raised up to the rated speed N, the ½ skip-firing intermittent operation and the ⅕ skip-firing intermittent operation are incorporated according to FIG. 5, while 3 minutes idling operation is placed on a part way in the starting steps. However, there is no feedback of engine operation conditions on the series of the starting steps; that is, there is no reflection of the engine conditions on the parameters (e.g., to determine n/m or 1/m based on real-time engine conditions) of the skip-firing intermittent operations. Further, the conventional manner according to FIG. 5 is not sufficiently useful to reduce the idling time span although the time reduction is desired.